1. Field of the Invention
The present invention relates to film labelstock, and particularly to extrudable film labelstock for in-mold labeling purposes, wherein good die-cuttability, printability, and/or contact clarity are desirable.
2. Brief Description of the Prior Art
The conversion of labelstock into labels for in-mold labeling involves the printing, sheeting, stacking, and die-cutting of the labelstock on a high type die cutter. A number of commercially desirable filmstocks exhibit an economically significant degree of edge welding between adjacent stacked sheets of film in die-cutting on such apparatus, however, such that individual labels cannot be properly formed and dispensed in the in-mold labeling process.
Consequently, polyethylene and a number of related highly extensible, inexpensive film materials have not heretofore proven to be economically useful alternatives for in-mold labeling, unless modified in some fashion so that the filmstock to be die-cut possesses a tensile ultimate elongation in both the machine and tensile directions, as determined by ASTM D-882, on the order of about 50% or less. It is noted that in fact some labelstocks, notably some polystyrene labelstocks, have die cut acceptably at the label thicknesses typically employed (i.e., 3-4 mils) in the in-mold labeling industry even with tensile ultimate elongations of from 45-50%, but for labelstocks generally it is considered that the lower figures represent an acceptable degree of extensibility at these typical thicknesses and in the commercial in-mold labeling processes.
Usually the films displaying a tensile ultimate elongation of this magnitude, i.e., the films which have to date proven sufficiently resistant to edge-welding to be of practical use in an in-mold labeling process of the type described, are either very highly biaxially oriented, or are highly filled with inorganic fillers, or possess a voided structure. Achieving a high degree of biaxial orientation requires comparatively expensive machinery relative to conventional blown and cast film apparatus. Highly filled filmstocks and those with a voided structure generally have a roughened surface which is not amenable to printing and exhibit poor contact clarity.
Recently-issued U.S. Pat. No. 4,883,697 (the '697 patent) to Dornbusch et. al. discloses a label for in-mold labeling wherein a thermoplastic label stratum is joined by coextrusion or preferably by an intervening adhesive layer to a stress-compensating stratum. The stress-compensating stratum is coated with a low temperature sealant layer such a the ethylene-vinyl acetate copolymer-paraffin-microcrystalline wax mixture described in U.S. Pat. No. 3,753,125 to Elliott. The thermoplastic label stratum can be polypropylene or other materials having a Young's modulus of greater than 100,000 psi, while the stress-accommodating stratum can be formed from materials such as low density polyethylene or ethylene-vinyl acetate copolymers having a Young's modulus of less than 40,000 psi.
Recently-issued and commonly-assigned U.S. Pat. No. 4,387,075 to Dudley discloses a coextruded label for use in in-mold labeling also, but relies on a surface printable layer of polystyrene.
While it is considered that films of the type described in the U.S. Pat. No. 4,883,697 would possible die cut acceptably and without a significant degree of edge welding, the patent does not teach or suggest an acceptable, wholly extrudable film labelstock which utilizes the edge-welding but inexpensive materials such as high density polyethylene, and which further presents a smooth surface for printing and excellent contact clarity.
Such a labelstock would be desirable from the perspective of enabling the economical use in an in-mold labeling process of materials which, despite their attractiveness as relatively inexpensive and effective label materials, previously have not been generally so used because of edge-welding between stacked sheets of the materials in die-cutting on a high type die cutter. The labelstock would further prove advantageous in being wholly extrudable rather than requiring an additional coating step, and by conventional blown and cast film processes and equipment. The absence of fillers in the labelstock such as would create a rough surface for printing or cause poor contact clarity in the labels would be a further desirable aspect.